This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-070944, filed Mar. 14, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a magnetic resonance imaging apparatus using a multi-echo fast imaging method represented by FSE (Fast Spin Echo) or EPI (Echo Planar Imaging).
EPI is an imaging method of continuously generating echoes by alternating the polarities of gradient fields and acquiring all data required to reconstruct one image by one excitation. This method completes imaging of one image in 50 ms, and hence can instantaneously image a moving organ such as the heart. In general, a series of echoes continuously generated after one excitation are called multiple echoes or an echo train.
As is known, the Larmor frequency of a specific proton of a region of interest is proportional to the magnetic field strength at the position of the proton. As described above, in EPI, gradient fields whose polarities alternate are used to generate multiple echoes. The direction of the positional shift of odd-numbered-echoes due to static field inhomogeneity becomes opposite to that of even-numbered-echoes. This tends to produce N/2 artifacts. Note that N is the number of encoding steps.
FIGS. 1, 2, and 3 are views for explaining N/2 artifacts. First of all, as shown in FIG. 1, odd-numbered-echoes and even-numbered-echoes are separately subjected to Fourier transform. Since the number of encoding steps is reduced to xc2xd, aliasing occurs. As a result, aliasing images (false images) are produced at a distance corresponding to xc2xd the number N of data. This false image is theoretically eliminated by adding an image generated from the odd-numbered-echo and an image generated from the even-numbered-echo, as shown in FIG. 2.
In practice, however, the inhomogeneity of static fields influences odd-numbered-echoes in the opposite direction to even-numbered-echoes with respect to the frequency axis, and hence opposite positional shifts occur along the frequency axis. As a consequence, the false image is partly left in the image obtained by the above addition, as shown in FIG. 3. This is so-called N/2 artifacts.
Such N/2 artifacts are caused by various uncertain factors such as the instability of the apparatus and the like as well as the above magnetic field inhomogeneity, it is practically impossible to completely eliminate them. Various methods of eliminating N/2 artifacts by software processing have also been proposed. However, these methods require repetitive processing and take much processing time, and a portion having no aliasing is required on an image, thus imposing limitations on practical use. Furthermore, they have not been proposed as methods that can also reduce blood flow artifacts.
The relationship between a fast imaging method using multiple echoes and blood flow artifacts will be described next. Blood flow imaging is a kind of MR image forming method of forming an image with sensitivity controlled with respect to a flow by using a phenomenon in which a spin that moves in a gradient field has a phase difference with respect to a stationary spin.
In a multi-echo fast imaging method such as EPI or FSE, blood flow artifacts tend to occur. In the multi-echo imaging method, a phenomenon called even-echo rephasing (to be referred to as EER hereinafter) occurs. As is known, this is a phenomenon in which the phases of even-numbered-echoes are refocused whereas the phases of odd-numbered-echoes are shifted and dephased. If, therefore, an image is reconstructed by using both even-numbered-echoes and odd-numbered-echoes, since a change in phase occurs every other echo resulting in blood flow artifacts.
As a method of solving both this problem of blood flow artifacts and the problem of the above N/2 artifacts, a flyback method using only odd-numbered-echoes or even-numbered-echoes has been proposed. In this method, however, since only about half of the echoes are used, the data acquisition efficiency deteriorates, and the imaging time (data acquisition time) is prolonged. In addition, the spatial resolution decreases as the imaging time remains unchanged.
As a method of speeding up almost all magnetic resonance imaging methods, a reconstructing method using the sensitivity distributions of multiple RF coils has recently received a great deal of attention (see 10th Ann. Scientific Meeting SMRM, 1240, 1991). According to this method, imaging is performed upon reduction of the number of encoding steps required to reconstruct one image, and the resultant aliasing is decomposed by using the difference in sensitivity distribution among the multiple RF coils, thereby obtaining an image without aliasing. As compared with general imaging methods, in this method, since the number of encoding steps can be basically decreased in inverse proportion to the number of coils, the imaging time can be shortened. This method will be briefly described below.
Assume that the number of multiple coils is two, a desired image is represented by I0(x,y), images originating from the respective coils are represented by I1(x,y) and I2 (x,y), and the sensitivity distributions of the respective RF coils are represented by S1(x,y) and S2(x,y). Assume also that the length of an imaging area in the encoding direction is represented by D, and imaging is performed upon reduction of the number of encoding steps to xc2xd so as to thin out every other line on the K-space. In this case, D/2 aliasing occurs, and the images I1(x,y) and I2(x,y) in which aliasing has occurred are written as follows:       (                                        I1            ⁡                          (                              x                ,                y                            )                                                                        I2            ⁡                          (                              x                ,                y                            )                                            )    =            (                                                  S1              ⁡                              (                                  x                  ,                  Y                                )                                                                        S1              ⁡                              (                                                      x                    +                    D                                    ,                  y                                )                                                                                        S2              ⁡                              (                                  x                  ,                  Y                                )                                                                        S2              ⁡                              (                                                      x                    +                    D                                    ,                  y                                )                                                        )        xc3x97          (                                                  I0              ⁡                              (                                  x                  ,                  y                                )                                                                                        I0              ⁡                              (                                                      x                    +                    D                                    ,                  y                                )                                                        )      
Therefore, the image I0(x,y) can be obtained by multiplying an inverse matrix of S as follows:       (                                        I0            ⁡                          (                              x                ,                y                            )                                                                        I0            ⁡                          (                                                x                  +                  D                                ,                y                            )                                            )    =                    (                                                            S1                ⁡                                  (                                      x                    ,                    y                                    )                                                                                    S1                ⁡                                  (                                                            x                      +                      D                                        ,                    y                                    )                                                                                                        S2                ⁡                                  (                                      x                    ,                    y                                    )                                                                                    S2                ⁡                                  (                                                            x                      +                      D                                        ,                    y                                    )                                                                    )                    -        1              xc3x97          (                                                  I1              ⁡                              (                                  x                  ,                  y                                )                                                                                        I2              ⁡                              (                                  x                  ,                  y                                )                                                        )      
In general, when N coils are used, an image is expressed by a matrix of Nxc3x97N. In the multiple echo imaging, even with the use of this reconstructing method using the sensitivity distributions of multiple RF coils, odd-numbered-echoes are used with even-numbered-echoes. Therefore the phase difference between odd-numbered-echoes and even-numbered-echoes is maintained, the blood flow artifacts in multiple echoes cannot be reduced.
As described above, in the EPI method, since data acquisition and reconstruction are performed by causing multiple echoes by using gradient fields having alternating polarities, N/2 artifacts tend to remain. In a fast imaging method using multiple echoes, e.g., EPI or FSE, since even-numbered-echoes differ in phase from odd-numbered-echoes, blood flow artifacts tend to occur.
It is an object of the present invention to remove N/2 artifacts, reduce blood flow artifacts, and improve blood flow extraction performance without sacrificing imaging time and S/N ratio in an imaging method using multiple echoes.
A magnetic resonance imaging apparatus includes a transmitting coil configured to generate RF pulses to a subject to be examined which is placed in a static field, a gradient coil configured to generate gradient magnetic field pulses, and a sequencer configured to control the transmitting coil and the gradient coil to continuously generate a plurality of echoes in accordance with a predetermined pulse sequence. The echoes are received by independent receiving coils. A computer generates a plurality of odd-numbered-echo-images respectively corresponding to the receiving coils on the basis of odd-numbered-echoes in the echoes, generates even-numbed-echo images respectively corresponding to the receiving coils on the basis of even-numbered-echoes in the echoes. The computer generates a first image without aliasing by unfolding the odd-numbered-echo-images on the basis of sensitivity distributions of the receiving coils. The computer generates a second image without aliasing by unfolding the even-numbered-echo-images on the basis of sensitivity distributions of the receiving coils. The computer generates a second image without aliasing by unfolding the even-numbered-echo-images on the basis of sensitivity distributions of the receiving coils.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.